Drilling Fluid: mixing in the correct order It is essential that products be mixed in the correct order. Some products have an adverse effect on other products when they are mixed in an incorrect order. For example, if PHPA polymers are mixed before the bentonite then the bentonite will not yield properly. The following mixing guidelines should be adhered to when mixing products. If there is a water quality problem, treat the water before you add any products. Add the required amount of bentonite and mix for 5-10 minutes. This should be adequate if the mixing system has sufficient shear. I would recommend mixing longer if the mixing system shear is not adequate. The fluid should look smooth and not have any large lumps floating around. In general, the longer you mix any fluid products the better they will work. One sack products that have polymer additives will need to be mixed longer than high yield bentonite products. After the bentonite is hydrated, add any polymer additives that you might be using. Here again, the mixing time should be extended to allow the polymers to hydrate and shear. The last addition should be lubricants or drilling detergent. Only mix long enough to make sure the additives are homogeneous in the fluid. Over mixing may cause a foaming problem, especially in the case of drilling detergent. Caution: If you are using a PHPA polymer in the fluid make sure you rinse out the mixing tank before you mix another batch of fluid. PHPA polymer is an effective flocculent in dilute solution. If there is a residue of polymer left in the tank it will flocculate the bentonite that you try to mix. Rinsing out the tank is a good practice to implement and takes very little time. SUMMARY Recommending a drilling fluid system should be based on the ability of the fluid to achieve these critical functions and to minimize problems. Initially, anticipated problems helps in selecting a particular drilling fluid system but other considerations may exist that dictate use of a different system.  The cost, availability of products and environmental factors are always important considerations. Drilling fluids almost always require tradeoffs in treating and maintaining the properties needed to accomplish the required functions.  For example, a high mud viscosity might improve hole cleaning, yet it might lower hydraulic efficiency, increase solids retention and slow the penetration rate. A driller that understands fluids and knows how to change them will have a powerful drilling tool at his disposal. Bentonite slurries will continue as the fluid of choice for most contractors. The slurry will be effective for most short, shallow and small diameter bores. Bear in mind the shortcomings that are inherent in bentonite slurries and be prepared to use additives when needed.  As the bores become larger and longer, the contractor should consider using a fluid system in place of the bentonite slurries. When the geological sequence also becomes challenging then fluid systems are demanded.